O'Conner One Rep Max Calculator

The O'Conner formula

1RM = w × (1 + 0.025 × reps), where w is the weight lifted and reps the number completed. Best suited to low reps; understates at high reps.

Worked example

Bench 100 kg for 10 reps. O'Conner: 100 × (1 + 0.025 × 10) = 100 × 1.25 = 125 kg. For 100 kg × 5: 100 × 1.125 = 112.5 kg — the same as Brzycki at that rep count.

How it compares

O'Conner and Brzycki agree closely up to about 6 reps. Past that O'Conner falls behind the pack: at 12 reps it gives 130 kg where Epley gives 140 and Wathan 142. Use it for low-rep sets or when you want a deliberately cautious number; switch to Wathan or Mayhew above 10 reps.

Which formula for which rep range

Rule of thumb: Brzycki, Lander and O'Conner for sets of 10 reps or fewer; Wathan and Mayhew when the set ran longer; Lombardi as a conservative all-rounder. The safest input is a hard set of 3 to 6 reps, where every formula nearly agrees.

The other 1RM formulas

• Epley formula — the default, and the parent calculator.
• Brzycki — 1RM = weight × 36 ÷ (37 − reps). The other standard; the most accurate of the set at low reps.
• Lombardi — 1RM = weight × reps^0.10. A power curve rather than a straight line — stays sane at high reps.
• Wathan — 1RM = 100w ÷ (48.8 + 53.8 × e^(−0.075 × reps)). An exponential fit that holds up well at higher reps.
• Lander — 1RM = 100w ÷ (101.3 − 2.67123 × reps). Close to Brzycki in shape; undefined near 38 reps.
• Mayhew — 1RM = 100w ÷ (52.2 + 41.9 × e^(−0.055 × reps)). Derived from bench-press data; a solid pick for higher reps.

Using the number

Once you have a 1RM, programmes prescribe work as a percentage of it — use the table above the fold to plan a session. Feed your bodyweight and goal into the calorie calculator and macro calculator to support the training, and see progressive overload for adding weight over time.

Source: O'Conner et al., 1989; see Wikipedia, "One-repetition maximum".